You can open the boot partition on the SD card (the FAT32 partition) and create an empty file named ssh in the root directory of that partition. Ensure that the file is names ssh and not ssh.txt !
If you are in the correct working directory in the command line, use
touch ssh
On recent Ubuntu version, this will switch to the correct directory and create the file (but you need to mount the directory manually e.g. using your file explorer:
cd /media/$USER/boot && touch ssh
Don’t forget to unmount the boot drive before removing the SD card. Once you restart the Raspberry Pi with the modified SD card, SSH will be enabled without you having to attach a keyoard or screen to the Pi.
This approach was tested with the 2018-11-13 version of Raspbian and works with Raspberry Pi 1, Raspberry Pi 2 and Raspberry Pi 3.
Credits to Yahor for the original solution on StackOverflow!